A201168.gs

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;"4"
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~:N
# Build up
1,](({
	# Array of boxed-123-avoiding arrays of length i.
	# Consider possible insertions of i into each array.
	# Since i will be the largest, we can neglect substitution by removing the largest.
	:A
	{
		.,:I,1/{
			# arr [val]
			.2$@/\I+*
		}%
		I@+\~
	}%

	# Filter
	{
		# array
		[
			.({1$<-}+%
			1$){1$<-}+%
			@0-{(}%
		]
		# [array-least array-leftmost array-rightmost] all renumbered
		A-!
	},
	[3,]-
}*

`#,